1. Field of the Invention
The present invention relates to an optical recording medium and an apparatus for optically reproducing (retrieving, playing back) information recorded on the optical recording medium.
2. Description of Related Art
FIG. 1 of the accompanying drawings illustrates a configuration of a recording surface of an optical disc, which is a conventional optical recording medium reproduced with sampled servo control.
As illustrated in FIG. 1, servo regions for recording various kinds of pits as servo references for a reproduction apparatus and data regions for recording digital data are alternately provided on respective recording tracks of the optical disc. The servo and data regions are aligned in a radial direction of the disc.
Referring to FIG. 2, illustrated is a detailed arrangement of the pits in the servo region.
As depicted in this drawing, the recording tracks are spaced from each other by a track pitch Tp. Two wobble pits Pwbl are formed in each of the servo regions along each of the recording tracks, and serve as references for the reproduction apparatus when the reproduction apparatus performs the tracking servo. One of the wobble pits in each pair is located above the associated recording track at a predetermined distance, and the other wobble pit is located below the recording track at the same distance. A clock pit Pclk is formed in each servo region on each recording track and serves as a reference for the reproduction apparatus when performing clock retrieval. A mirror portion between the wobble pits Pwbl and clock pit Pclk is used as a synchronization reference for the reproduction apparatus.
When the recorded information should be read out from the optical disc having the above described structure, a pickup installed in the reproduction apparatus radiates a scanning (reading) laser beam onto the recording surface of the optical disc to create a beam spot BS on the recording surface as shown in FIG. 2. As the optical disc rotates, the beam spot BS traces the recording track indicated by the solid line in FIG. 2 from the left to the right. The pickup receives reflected light of the beam spot BS from the recording surface and obtains a scanned (read) signal in accordance to an amount of the reflected light. During this operation, the tracking servo is conducted in order for the beam spot BS to accurately trace the recording track. To this end, the beam spot BS is required to trace the recording track that extends on the center line of the two wobble pits Pwbl. The tracking servo is therefore performed in such a way that scan signals resulting from the two wobble pits Pwbl have the same level.
The diameter D of the beam spot BS is proportional to a wavelength xcex of a scanning laser beam and inversely proportional to the numerical aperture NA of an objective lens. In other words, the following equation is established:
D=kxc2x7xcex/NA (k is a proportion constant)
In general, the diameter D of the beam spot BS is defined by a circle diameter at which the light intensity becomes 1/e2 of the light intensity at the beam spot center. k=0.82 is under this condition so that the above equation becomes as follows:
D=0.82xc2x7xcex/NA
The optical disc of the conventional sampled servo scheme and the apparatus for reproducing the recorded information from such optical disc have the following values:
xcex=780 [nm]
NA=0.5
D=1279 [nm]
Tp=1600 [nm]
Accordingly, the track pitch Tp shown in FIG. 2 is greater than the diameter D of the beam spot BS. In addition, although the interval (distance) L between a pair of wobble pits Pwbl in the information scanning direction varies with the disc radius as understood from FIG. 2, the wobble pit interval L is 2030 [nm] when measured at the disc radius of 30 [nm]. Thus, the wobble pit interval L is greater than the diameter D of the beam spot BS.
The above described conventional optical disc has the track pitch Tp greater than the beam spot diameter D so that recording density is low in the disc radius direction, and has the wobble pit interval L greater than the beam spot diameter D so that recording density is low in the recording track direction. Consequently, the recording density of the conventional optical disc is low on the recording surface.
Furthermore, the conventional optical disc needs to record the wobble pits at positions spaced from the recording track in the disc radius direction in order for the reproduction apparatus to conduct the tracking servo.
Therefore, a recording apparatus requires a deflection device to deflect a recording laser beam in the disc radius direction when the recording apparatus records the wobble pits. As a result, the recording apparatus has a complicated structure.
The present invention was developed to overcome the above described problems, and its primary object is to provide an optical recording (storage) medium that permits high density recording and allows use of a recording apparatus having a simpler structure, and to provide an apparatus for optically reproducing recorded information from such optical recording medium.
According to one aspect of the present invention, there is provided an optical recording medium comprising recording tracks formed on a recording surface of the optical recording medium, and tracking pits for tracking servo control formed on the recording surface of the optical recording medium, wherein a track pitch of the recording tracks is smaller than a diameter of a beam spot of a scanning laser beam radiated onto the recording medium, the tracking pits are formed on the recording tracks, a plurality of neighboring recording tracks define a group, and the tracking pits in each group are spaced from each other in a recording track direction by a predetermined distance which is smaller than the diameter of the beam spot.
According to another aspect of the present invention, there is provided an apparatus for optically reproducing recorded information from an optical recording medium, the optical recording medium having tracking pits for tracking servo control formed on recording tracks, comprising a pickup for radiating a scanning laser beam onto the optical recording medium to obtain read signals in accordance with a reflected light of the scanning laser beam, an A/D converter for sampling the read signals to convert them into a series of sampled values, and tracking error detecting part for extracting two sampled values corresponding to tracking pits formed on two adjacent recording tracks from the series of sampled values to take a level difference between the extracted sampled values as. a tracking error.